1. Field of the Invention
The invention generally relates to a sealing device for turbine engines. Specifically, the invention is a segmented intershaft seal with a T-shaped cross section and centrifugal compensation. The segmented intershaft seal is disposed between inner and outer shafts. Each seal segment is biased in an outward radial direction so as to initially contact the outer shaft. Hydrodynamic pockets along the outer shaft form a thin-film layer between the outer shaft and the seal segments when the inner and outer shafts rotate. The centrifugal compensation offsets the outward centrifugal forces acting on the seal segments so as to allow formation of a thin-film layer and separation of the seal segments from the outer shaft.
2. Background
Intershaft seals are often employed between counter-rotating or co-rotating shafts within a turbine engine. Intershaft seals minimize wear and friction induced heat by rotating with one shaft immediately adjacent to and separate from another shaft. As such, intershaft seals are typically referred to as clearance-type seals. In one example, a sealing element could contact and rotate with an inner shaft without contacting the outer shaft. In another example, a sealing element could contact and rotated with an outer shaft without contacting the inner shaft. Seal clearance can be in either radial or axial directions. In both examples, contact induced wear is minimized by limiting interactions between seal and opposing shaft to those caused by heat or thrust induced misalignments.
In some turbine engines, operational parameters could limit fluid flow between inner and outer shafts such that clearance-type seals are inadequate and could also limit wear along the sealing element such that contact-type seals are not practical. In these applications, a hybrid approach is required whereby the sealing element contacts the opposing shaft below a predetermined threshold, typically related to the rotational speed of the shaft(s), and separates from the opposing shaft above the threshold. Below the threshold, the sealing element prevents fluid from flowing between the shafts and minimizes wear along the sealing element. Above the threshold, the sealing element limits flow of fluid between the shafts and avoids wear along the sealing element.
Hydrodynamic pockets are often employed to form a thin-film layer between a sealing element and a shaft so as to separate the sealing element which otherwise contacts the shaft. Unfortunately, the lift force generated by hydrodynamic pockets is limited and not sufficient to overcome the centrifugal forces acting on a sealing element disposed along an inner shaft. One solution is centrifugal compensation to offset, negate, or cancel the outward centrifugal forces acting on the sealing element that otherwise frustrate hydrodynamic sealing.
Stein describes a seal in U.S. Pat. No. 4,211,424, entitled Centrifugal Compensated Seal for Sealing between Concentric Shafts, for sealing between a hollow outer shaft and an inner shaft concentric with the outer shaft having a seal ring with a plurality of segments extending around the inner shaft. The seal ring has an outer circumferential surface confronting the inner surface of the outer shaft, a first side face exposed to a region of high pressure and a second side face confronting a mating ring secured to the inner shaft and having a portion thereof exposed to a region of lower pressure. A segmented compensating ring extends around the inner shaft and spaced from the seal ring in the region of high pressure. A flange secured to the inner shaft limits the axial movement of the compensating ring away from the seal ring. A continuous balancing ring between the seal ring and the compensating ring has a pair of conical faces mating with conical faces on the seal ring and compensating ring, respectively. Hydrodynamic and closed pockets in the bearing surfaces are used to vary the contact forces.
Stein states that his “invention relieves the centrifugal loading with relatively simple means without resorting to a multiplicity of hinged or articulated counterweights or similar complex mechanisms.” In doing so, Stein teaches away from solutions including hinged or articulated counterweights without specifically describing such mechanisms.
The complexity of a centrifugal compensation mechanism influences the functionality, assemble-ability, and reliability of a sealing system. Stein infers that hinged or articulated solutions result in functionally deficient, difficult to assembly, and unreliable sealing systems.
Accordingly, what is required is a sealing element including hinged counterweights which is functional, easily assembled, and reliable so as to provide contact and non-contact sealing during operation of a turbine engine.